The decision she had to make was not did Brian hit Sami high, that was never in doubt. It was did the high shot cause Sami's foot to land on the sideline in her opinion. I don't have an opinion on that as my opinion is irrelevant, but hers which was relevant was that it didn't and seems reasonable having looked at it repeatedly. The difference it made in the game was minute anyway, as they scored almost immediately and got the kick. It would have added a minute onto the game, that's all.
I do have an issue with people in the NRL coming out and saying she got this wrong, as the rules say "in the opinion of the referee" so it was her opinion, not anyone else's. I do think she got the Papalii knock on wrong though, but that wasn't a subjective opinion, it was clearly kicked out by a player who hadn't cleared the ruck, and I agree with you that she makes too many errors but she's not alone there.